#78 Monster AI should take region into account

[Andrew Fuchs]

To allow monsters to be placed onto the world map more
freely, their AI should account for regions in some way.

This could be done by placing three additional fields
into the region file entries, "monster_avoid" and
"monster_constrain". Theis fields should be able to
contain names of monsters, races, or only in the case
of "monster_avoid" a maximum level. "monster_avoid"
would stop all monsters from moving into maps in that
region, or if the monster is on such a map it would try
to get off of it. "monster_constrain" would stop the
AI from moving onto any map not in that region. I have
no idea what should happen if the monster is pushed out
of a region it is constrained to.

All of this should take the nesting of regions into
account.